1. Field of the Invention
The present invention relates to a double power sources switching circuit, and more particularly to a double power sources switching circuit of a product with a USB device capable of being connected to a USB port of a personal computer.
2. Description of Related Art
A general product with a USB device can be connected to a personal computer through a USB port of the personal computer. For example, a power source switching system and method of an external connection CD-ROM drive disclosed in Taiwan Patent Publish No. 200609743 applies hardware and firmware technologies to allow the external connection CD_ROM drive to receive not only domestic power source but also power source provided by a computer mainframe through an USB.
USB ports of current notebook computer and a several desktop personal computer cannot obtain power supply when they are in a power-down or sleep state, a product with double power sources USB device will switch the USB port to an internal power source to provide power source needed for it at this moment. But, because the discharge time of the USB port of the computer is too long in this process, it will lead to a situation that an uncertain section appears in the period of power source switching to cause the product in use to be reset or locked to cause the user's data to be lost and inconvenience to the user.